US20100188089A1 - Multiple Receiver Coil System For Geophysical Prospecting - Google Patents
Multiple Receiver Coil System For Geophysical Prospecting Download PDFInfo
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- US20100188089A1 US20100188089A1 US12/645,961 US64596109A US2010188089A1 US 20100188089 A1 US20100188089 A1 US 20100188089A1 US 64596109 A US64596109 A US 64596109A US 2010188089 A1 US2010188089 A1 US 2010188089A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
- G01V3/15—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for use during transport, e.g. by a person, vehicle or boat
- G01V3/165—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for use during transport, e.g. by a person, vehicle or boat operating with magnetic or electric fields produced or modified by the object or by the detecting device
Definitions
- FIGS. 2A , 2 B and 2 C are each perspective views of the receiver coil assembly of FIG. 1 , each highlighting a respective one of three receiver coils of the coil assembly.
- Elastic members 32 can be formed from rubber or other suitable elastic or resilient material.
- the fastening assembly 40 could take many configurations different from that shown in FIGS. 4 and 5 to elastically suspend the inner frame member 12 A.
- the skeletal octahedronal receiver coil assembly 10 provides relatively light weight structure for housing and supporting the three receiver coils 16 A, 16 B and 16 C in different planes about a common central point. Furthermore, the skeletal nature of the receiver coil assembly can mitigate the drag caused by a fluid such as air or water passing through the assembly 10 when compared for example to a solid spherical tow assembly.
- the elasticised suspension of the internal frame 20 can in at least some applications mitigate against noise causing vibrations that the receiver coils may otherwise be subjected to.
- the octahedronal structure of skeletal frame 8 can in at least some example embodiments provide a strong structure for maintaining the receiver coils 16 A, 16 B and 16 C in substantially stable positions relative to each other.
- FIG. 7B illustrates another possible tow cable configuration for towing the tow assembly 10 from an aircraft or other carrier.
- a separate connection rope 66 extends from each of the corner sections 18 of the receiver coil assembly 10 to a central hub connection 68 that is secured to a tow rope 64 .
- the lengths of at least some of the respective connecting ropes 66 may be different to provide a desired flight orientation for the receiver coil assembly.
- the connecting ropes 66 may also apply tension to the respective corner sections 18 and thereby add strength and rigidity to the receiver coil assembly 10 .
- the tow cable configuration can vary from that shown in FIG. 7 depending on the application—for example the assembly could alternatively be suspended from a net or connection ropes 66 connected to portions of the frame 8 other than or in addition to the corners 18 .
Abstract
Description
- This application claims the benefit of and priority to: U.S. provisional patent application Ser. No. 61/140,271 filed Dec. 23, 2008; U.S. provisional patent application Ser. No. 61/154,024 filed Feb. 20, 2009; and U.S. provisional patent application No. 61/264,687 filed Nov. 26, 2009, the contents of which are incorporated herein by reference.
- This description relates to a multiple receiver coil system and apparatus for geophysical surveying.
- Geophysical electromagnetic (“EM”) prospecting techniques can be effective in determining the electrical conductivity of soils, rocks, and other bodies at and under the earth's surface.
- Geophysical EM prospecting can be carried out using surface based equipment and airborne equipment. Airborne methods in which equipment is transported by aircraft such as helicopter, airplane or airship may be useful for large area surveys. For airborne electromagnetic (“AEM”) systems, survey data may be acquired while an airplane or helicopter flies at a nearly constant speed along nearly-parallel and close to equally-spaced lines at an approximately constant height above ground. In some applications, geophysical EM prospecting of a seabed may be carried out using equipment located under the surface of a body of water.
- Some geophysical surveying methods are active in that the equipment is used to transmit a signal to a targeted area, and then measure a response to the transmitted signal. Other geophysical surveying methods are passive in that signals produced from a target area are measured without first transmitting a signal to the target area.
- An example of a passive geophysical EM prospecting method is Audio Frequency Magnetic (“AFMAG”) surveying in which the EM fields resulting from naturally occurring primary signal sources such as lightning discharges are measured. These EM fields propagate around the earth as plane waves guided by the ionosphere and earth's surface. Lightning activity occurring remote from the measurement point can produce signals with a nearly flat spectral density at frequencies between, for example, 8 Hz and 500 Hz, varying with geographical location, time of the day, seasons and weather conditions. An example of a passive AFMAG geophysical EM prospecting method is shown in U.S. Pat. No. 6,876,202.
- Examples of active geophysical EM prospecting methods include methods in which a transmitter is used to illuminate a target area with a primary field and a receiver is used to measure the secondary field generated by the target area. Such systems are often frequency domain or time domain systems. In at least some frequency-domain electromagnetic (“FDEM”) systems, a transmitter coil continuously transmits an electromagnetic signal at fixed multiple frequencies, while the receiver coil measures the secondary field signals continuously over time.
- In at least some time-domain electromagnetic (“TDEM”) systems, a pulse of current is applied to a transmitter coil during an on-period and switched off during the off-period, typically at a repetition rate equal to an odd multiple of half of the local power line frequency. A response signal is measured at a receiver as a function of time. The response signal amplitude decay during the off-period, combined with modeling of the conductivity and geometry of geological bodies in the ground, can be utilized to yield the conductivity contour maps. An example of a TDEM system is shown in U.S. Pat. No. 7,157,91.
- According to one example embodiment there is provided a receiver coil tow assembly for geophysical prospecting, comprising: multiple receiver coils, each receiver coil being housed within a respective tubular outer frame section that defines a continuous passageway in which the receiver coil extends, the tubular outer frame sections being connected together to provide a skeletal frame maintaining the receiver coils in a substantially constant position relative to each other; and a tow cable connected to the skeletal frame for towing the frame to conduct a geophysical survey.
- According to one example embodiment there is provided a receiver coil tow assembly for airborne geophysical prospecting comprising: first, second and third tubular frame sections, each of the frame sections forming a loop defining a respective internal passage, each of the first, second and third tubular frame sections being connected to the other tubular frame sections at spaced apart locations to form a tow assembly frame in which the first, second and third tubular frame sections are maintained in a substantially constant position relative to each other; and first, second and third receiver coils for measuring magnetic field signals, the first, second and third receiver coils being housed within the internal passages of the first, second and third tubular frame sections respectively with the first, second and third receiver coils each having an orientation different than that of the other receiver coils.
- According to one example embodiment there is provided a receiver coil assembly comprising an outer tubular frame forming a loop and defining an internal passageway; an inner frame extending within the internal passageway; an receiver coil for measuring magnetic field signals and secured to the inner frame; a first and second plurality of elastic members elastically suspending the inner frame within the internal passageway so as to apply opposing forces in at least two directions biasing the inner frame into a centrally within the internal passageway; and a first elongate rod member connected to a plurality of the first elastic members and a second elongate rod member connected to a plurality of the second elastic members, the elongate rod members enhancing isolation of the inner frame from vibration of the outer tubular frame.
- Example embodiments of the invention are provided in the following description. Such description makes reference to the annexed drawings wherein:
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FIG. 1 is a perspective view of an example embodiment of a multiple receiver coil assembly according to one example embodiment. -
FIGS. 2A , 2B and 2C are each perspective views of the receiver coil assembly ofFIG. 1 , each highlighting a respective one of three receiver coils of the coil assembly. -
FIG. 3 is a perspective view of an internal frame of the receiver coil assembly ofFIG. 1 . -
FIG. 4 is a sectional view of part of the receiver coil assembly, taken along the lines IV-IV ofFIG. 2A . -
FIG. 5 is a sectional view of part of the receiver coil assembly, taken along the lines V-V ofFIG. 4 . -
FIG. 6 is a perspective view of a corner section of the receiver coil assembly ofFIG. 1 . -
FIG. 7A is a perspective view showing a possible tow rope connection for the receiver coil assembly ofFIG. 1 . -
FIG. 7B is a perspective view showing a possible tow rope connection for the receiver coil assembly ofFIG. 1 . -
FIG. 8 is a perspective view of an alternative embodiment of a receiver coil assembly. -
FIG. 9 is a sectional view of part of the receiver coil assembly, according to an alternative embodiment, taken along the lines IV-IV ofFIG. 2A . -
FIG. 10A is a sectional view of part of the receiver coil assembly, according to an alternative embodiment, taken along the lines X-X ofFIG. 9 . -
FIG. 10B is a sectional view of part of the receiver coil assembly, according to another alternative embodiment. -
FIG. 10C is a sectional view of part of the receiver coil assembly taken at right angles to the sectional view ofFIG. 10B . -
FIG. 10D is a sectional view of part of the receiver coil assembly, according to another alternative embodiment. -
FIG. 11 shows a representation of an AFMAG geophysical prospecting system according to one example embodiment of the invention. -
FIG. 12 is a schematic view of the geophysical prospecting system ofFIG. 11 . -
FIG. 13 shows a representation of a geophysical prospecting system according to another example embodiment of the invention. -
FIG. 14 shows a representation of a geophysical prospecting system according to another example embodiment of the invention. -
FIG. 15 is another example embodiment of a possible skeletal frame for a receiver coil assembly. -
FIG. 16 is another example embodiment of a possible skeletal frame for a receiver coil assembly. -
FIG. 1 illustrates a multiplereceiver coil assembly 10 for geophysical surveying, according to example embodiments of the invention. Thereceiver coil assembly 10 includes multiple receiver coils supported within askeletal frame 8 that maintains the coils in a substantially fixed position relative to each other. As will be explained in greater detail below, in at least some configurations thereceiver coil assembly 10 can allow a relatively low weight to coil size ratio, and in applications where thereceiver coil assembly 10 is moved through a fluid such as air or water, can mitigate against drag. - The multiple
receiver coil assembly 10 ofFIG. 1 includes three air-core receiver coils 16A, 16B and 16C supported within theframe 8. Each of thecoils coils - In an example embodiment, the
skeletal frame 8 is made up of tubular members that define internal passages in which aninternal frame 20 is elastically suspended, and the receiver coils 16A, 16B and 16C are secured to theinternal frame 20. More particularly, in the illustrated embodiment, the skeletal frame includes three interconnecting tubularouter frame sections support frame sections support frame sections respective receiver coil support frame sections internal frame 20, as diagrammatically illustrated inFIG. 3 . Accordingly, each of the receiver coils 16A, 16B and 16C are substantially contained within a respectiveindependent frame section -
FIGS. 2A , 2B and 2C are provided to individually highlight theframe sections Frame sections respective receiver coil frame sections internal frame sections skeletal frame 8 is substantially shaped like a regular octahedron. As shown inFIG. 2A , theouter frame section 8A is formed by four elongatetubular frame members 14A connected by fourcorner sections 18 to form square-shapedframe section 8A which defines a continuousinternal passage 22A in which correspondingly square-shapedinternal frame section 20A is elastically suspended. Theinternal frame section 20A is formed by fourelongated frame members 12A connected by corner sections 24 (FIG. 3 ). The terms “internal” and “inner” are used interchangeably herein when referring to framesections frame members receiver coil 16A is secured within a trough or passage defined by theinternal frame section 20A. - Similarly, as shown in
FIG. 2B , thesub-frame 8B is formed by four elongatetubular frame members 14B connected by fourcorner sections 18 to form square-shapedframe section 8B which defines a continuousinternal passage 22B in which correspondingly square-shapedinternal frame section 20B is elastically suspended. Theinternal frame section 20B is formed by fourelongated frame members 12B connected bycorner sections 24. Thereceiver coil 16B is secured within a trough or passage defined by theinternal frame section 20B. As shown inFIG. 2C , thesub-frame 8C is formed by four elongatetubular frame members 14C connected by fourcorner sections 18 to form square-shapedframe section 8C which defines a continuousinternal passage 22C in which correspondingly square-shapedinternal frame section 20C is elastically suspended. Theinternal frame section 20C is formed by fourelongated frame members 12C connected bycorner sections 24. Thereceiver coil 16C is secured within a trough or passage defined by theinternal frame section 20C. - As can be seen in
FIGS. 1-3 , each of the outerframe corner sections 18 is shared by and interconnects a corner of two of thetubular frame sections frame corner sections 24 is shared by and interconnects a corner of two of thetubular frame sections - In the illustrated example embodiment, the receiver coils 16B and 16C and their respective supporting inner and
outer frame members receiver coil 16A and its supporting inner andouter frame members FIGS. 4 and 5 . As can be seen inFIG. 4 , in an example embodiment, eachtubular frame member 14A comprises two substantially identical elongate half-cylindrical sections 26 which are secured together by matingperipheral flange portions 28 to define the internalreceiver coil passage 22A.Bolts 30 or other fasteners can pass through aligned fastening holes 30 (FIG. 5 ) to secure the half-cylindrical sections 26 together. Half-cylindrical sections 26 may for example be formed from a light-weight rigid composite material that allows electromagnetic signals to pass through with minimal or no interference toreceiver coil 16A. Theinner frame member 12A is elastically suspended within thetubular frame member 14A approximately at the center of thecoil passage 22A bymultiple fastening assemblies 40 that are spaced internally along the length of each of thetubular frame members 14A. - As best seen in
FIG. 5 , eachfastening assembly 40 includes anelastic suspension member 32 that extends between the inner wall of thetubular frame member 14A and theinner frame member 12A. In one example embodiment, each elastic suspension member 32 (which may for example be formed from rubber) is secured at opposite first and second ends 38 to longitudinally spaced locations on a side of theinner frame member 12A, and at an approximate mid-point 36 to the inner wall of thetubular frame member 14A such that theelastic suspension member 32 forms a “V” shape that applies opposing longitudinal forces to theinner frame member 12A as well as a lateral force. (It will be appreciated that the “V” shaped elastic member could be replaced with two separate elastic pieces.) Afastening block 34 may be secured by adhesive or other fastener to the inner wall of thetubular frame member 14A to provide a surface for securing the mid-point 36 by a bolt or other fastener. In the illustrated embodiment,fastening assemblies 40 are located in pairs on opposite sides of theinner frame member 12A such that substantially equal but opposite forces are applied to theinner frame member 12A by theelastic suspension members 32 so that theinner frame member 12A normal resting position is in the center of thecoil passage 22A defined bytubular frame member 14A, regardless of the orientation of theframe 10. In one example embodiment, theelastic suspension members 32 in atubular frame member 14A are all secured to one half-cylindrical section 26 thereof to facilitate securing theinner frame member 12A in place before closing up thereceiver coil passage 22A with the other half-cylindrical section 26. -
Elastic members 32 can be formed from rubber or other suitable elastic or resilient material. Thefastening assembly 40 could take many configurations different from that shown inFIGS. 4 and 5 to elastically suspend theinner frame member 12A. - Referring again to
FIG. 4 , in some embodiments theinner frame member 12A has a V-shaped cross-section and defines an open-sided trough 42 that provides aninner cable passage 44 in which thereceiver coil 16A is received. In some example embodiments, theinner frame member 12A could alternatively have a semi-rectangular, or semi-circular or circular or other cross-sectional area. In at least some embodiments thereceiver coil 16A is a loop or multi-turn coil formed that is secured in thetrough 42 by tape and/or other type of fastening mechanism. - In the illustrated embodiment, the octahedronal
skeletal frame 8 includes a total of twelvetubular frame members members corner sections 18. Each of the corner sections joins a pair of the tubular frame members that support one of the receiver coils with a pair of the tubular frame members that support one of the other two receiver coils, such that portions of two receiver coils pass through each of thecorner sections 18.FIG. 6 illustrates, without showing any receiver coils, one of thecorner sections 18 in greater detail according to an example embodiment. Thecorner section 18 includes a removable inner wall section 62 (removed in FIG. 6—seeFIG. 2C ) and anouter basket section 48.Outer basket section 48 includes a semi-spherical central portion from which foursemi-cylindrical stubs 50 extend. Each of thestubs 50 has a lateralperipheral flange 52 for mating with a corresponding flange 60 (FIG. 2C ) on a correspondingtubular frame member holes 58 are provided along theflanges inner wall section 62 has a shape that corresponds to that of the outer basket section and includes peripheral flange portions that mate withflange portions 54 of the outer basket sections andflanges 60 of the correspondingtubular frame members inner frame 12 includesinner corner portion 24 that includes trough-definingarms 56 that are secured toinner frame members corner section 18. In some example embodiments, the innerframe corner portion 24 is secured to the outerframe basket section 48 and/or the removableinner wall section 62 by elastic members, however in some embodiments the innerframe corner portion 24 is only connected to and supported by the remainder of theinner frame 12. - In at least some configurations, the skeletal octahedronal
receiver coil assembly 10 provides relatively light weight structure for housing and supporting the threereceiver coils assembly 10 when compared for example to a solid spherical tow assembly. The elasticised suspension of theinternal frame 20 can in at least some applications mitigate against noise causing vibrations that the receiver coils may otherwise be subjected to. The octahedronal structure ofskeletal frame 8 can in at least some example embodiments provide a strong structure for maintaining the receiver coils 16A, 16B and 16C in substantially stable positions relative to each other. -
FIG. 7A illustrates one possible towing configuration for towing thetow assembly 10 from an aircraft or other carrier. In the illustrated example three connectingropes 80 have first ends that are each respectively connected to threeupper corners 18 of thetow assembly frame 8 and opposite ends that are connected to acommon connector 82. The first ends of the connectingropes 80 can for example be connected to connecting loops or eyes (not shown) provided on thecorners 18, or be tied directly to the corners of theframe 8. In some example embodiments thecommon connector 82 is connected by one or more bungee-type cords 84 to the end of atow rope 64, which is attached to an aircraft. Bungee cords 84 (or a suitable elastic alternative) can in some configurations assist in isolating thereceiver coil assembly 10 from aircraft vibrations. The lengths of the respective connectingropes 80 can be different from one rope to the next and can be selected to provide theframe 8 with different desired orientations at different horizontal flight speeds. For example, the connection rope lengths could be selected so that at a typical survey speed the receiver coils 8 have a certain orientation such as shown inFIG. 7A , but at low or no horizontal speed theframe 8 can be vertically lowered to land generally simultaneously on threelower corners 18 in the position shown inFIG. 1 to reduce landing and take-off stresses on the frame. In some embodiments one or more fins or baffles formed from fabric or other light-weight material can be selectively placed on one or more portions of theframe 8 to provide an air interface surface to result in a desired orientation of theframe 8 during flight. - In some example embodiments, pre-amplifiers are included in the frame assembly and connected to leads from the receiver coils 16A, 16B and 16C for amplifying the signals received by
receiver coils FIG. 7A ,pre-amplifiers 86 can be provided inside onecorner 18 of theframe 8 for the two receiver coils that pass through that corner (forexample coils further preamplifier 86 for the other receiver coil (forexample coil 16C) provided in inside anothercorner 18 of the frame. Ajunction box 108 located on one of thecorners 18 can be connected to each of the receiver coils 16A, 16B and 16C throughpre-amplifiers 86. Thejunction box 108 is in turn connected toelectrical cables 124 that extend adjacent one of the connectingcables 80 and along thetow rope 64 to a data monitoring computer that receives information from the receiver coils 16A, 16B and 16C, and a power source used for poweringpre-amps 86 and other active devices such as GPS receivers or other positional devices that may be attached to theframe 8. In some embodiments, leads from a pre-amplifier 86 located at onecorner 18 of the frame to thejunction box 108 may be provided internally within one of the tubular members of the frame, spatially separated from the receiver coil in that particular tubular member, as indicated by the dashedline 88 inFIG. 7A . - In some example embodiments
positional sensors 90 such as GPS sensors and/or accelerometers can be located at one or more locations of theframe 8—for example,FIG. 7A illustrates threeGPS antennas 90 located at threerespective corners 18 of theframe 8, which are electrically connected to aGPS receiver 92 that is mounted to theconnector 82 ortow cable 64. In some example embodiments, other positional technology could be mounted to theframe 8, for example the Novatel™ SPAN positioning technology such as the SPAN IMU-LN200 or SPAN CPT—on some cases a sub-frame may be attached toframe 8 to mount positioning technology at the center of the frame. -
FIG. 7B illustrates another possible tow cable configuration for towing thetow assembly 10 from an aircraft or other carrier. In the illustrated example, a separate connection rope 66 extends from each of thecorner sections 18 of thereceiver coil assembly 10 to a central hub connection 68 that is secured to atow rope 64. The lengths of at least some of the respective connecting ropes 66 may be different to provide a desired flight orientation for the receiver coil assembly. The connecting ropes 66 may also apply tension to therespective corner sections 18 and thereby add strength and rigidity to thereceiver coil assembly 10. The tow cable configuration can vary from that shown inFIG. 7 depending on the application—for example the assembly could alternatively be suspended from a net or connection ropes 66 connected to portions of theframe 8 other than or in addition to thecorners 18. - In some example embodiments, the tow cable configuration is selected (for example by connection cable length and location, and perhaps through the use one or more stabilizer fins attached to tow assembly 10) so that the receiver coil axes are substantially maintained in a desired orientation during flight, for example, one coil axis being in a vertical direction, one coils axis being oriented in a direction of travel, and one coil axis being horizontally oriented at a right angle to the direction of travel.
- In one example embodiment, electrical leads for each of the receiver coils 16A, 16B and 16C pass through respective openings provided in the
outer frame 8 and are connected to electrical cables that are secured to thetow rope 64 so that the receiver coils 16A, 16B and 16C can be remotely monitored from the towing aircraft. - As will be appreciated from the above description, in example embodiments the
coil assembly 10 is constructed in such a manner that it can be disassembled and transported and then reassembled at a survey location. As noted above, in example embodiments theinternal frame section 20A,internal frame section 20B andinternal frame section 20C are each formed by fourelongated frame members corner sections 24, and the receiver coils 16A, 16B and 16C are each secured by tape or adhesive or other fasteners within the respective coil channels orpassages 42 of theelongated frame members internal frame section 20A, in an example embodiment,corner sections 24 are releasably connected at the ends of the fourstraight frame members 12A that such that during disassembly of thetow assembly 10, the four straightelongated frame members 12A can be removed from thereceiver coil assembly 10 together with thereceiver coil 16A still secured toelongated frame members 12A such that thereceiver coil 16A will be substantially rigid along four sides but flexible at four corners, which allows the receiver coil (together with the four straightelongated frame members 12A) to be folded into a compact bundle in which the four straightelongated frame members 12A are all positioned parallel to each other while maintaining continuity of the conductors that make up the receiver coil and allowing thereceiver coil 16A to be subsequently reinstalled in thereceiver coil assembly 10 in a substantially similar configuration.Internal frame sections receiver coils - In some example embodiments, the
tubular frame members inner frame members FIG. 8 illustrates atubular frame member 14B that is formed from a plurality ofsub-sections 70 that are secured together at mating portions byfasteners 72. Segmented frame members can facilitate transportation of larger receiver coil assemblies to and from a survey location as kits that can be assembled and disassembled on location. Furthermore, the same frame members can be used to assemble different size frames 8. - Referring to
FIGS. 9 and 10A , in another alternative embodiment, a double suspension system is used to suspend the receiver coilinterior frame 20 within theouter frame 8 ofreceiver coil assembly 10. AlthoughFIGS. 9 and 10A illustrate theframe sections receiver coil 16A, theframe sections FIGS. 9 and 10A . In the double suspension configuration ofFIGS. 9 and 10A , thesuspension assemblies 40 located along the lengths of each of theinternal frame members intermediate frame members 74, which are in turn suspended from theouter frame members elastic suspension members 32 opposingly suspend theinner frame member 12A in the center of a cylindrical or semi-cylindricalintermediate member 74, which is then centrally suspended in a similar manner by furtherelastic members 76 that extend between the cylindricalintermediate member 74 and theouter frame member 14A. As can be seen inFIG. 10A , the furtherelastic members 76 can also be arranged in V-shaped pattern to act against longitudinal movement as well as radial movement in a similar manner as the firstelastic suspension members 32. - Thus, in the embodiment of
FIGS. 9 and 10A , theinner frame section 20A that supportsreceiver coil 16A is suspended by a number of firstelastic suspension members 32 from a number of respectiveintermediate frame sections 74 which are in turn suspended by one or more second elastic suspension members 76 (which may for example be formed from rubber) from theouter frame 8A. Theinner frame section 20A may further be positioned at or near the centre of the outer frame. Regions that are (i) proximate the connections between thefirst suspension members 32 and each of theinner frame section 20A and theintermediate frame sections 74, and (ii) proximate the connections between thesecond suspension members 76 and each of theintermediate frame sections 74 and theouter frame 8A can be coated with a friction reducing agent such as silicone. A silicone coating may reduce the noise caused by rubbing at the attachment or connection point. In some example embodiments, the first suspension members may be connected to the respective frame sections by cable ties that pass through pre-drilled holes or attached loops. Alternatively, any number of other possible methods can be used to attach the first and second suspension members including: hooks, or a machined hook-like attachment point connected to the attachment points whereby the suspension members may be looped around the hooks and then covered by silicone; alternatively, loops on the first and second suspension members can be screwed into the attachment points; another possibility is to glue the first and second suspension members to the inner frame, and to the outer frame or intermediate frame sections. - As shown in
FIGS. 9 and 10A both the first andsecond suspension members inner frame sections receiver coil passages receiver coil assembly 10 on the receiver coils 16A, 16B and 16C. In other example embodiments, other support mechanisms can be used including triple-suspension, springs, surrounding the coil with foam, or other means of positioning the coil in the centre of the inner frame in a manner that reduces noise. - In some embodiments the location and positioning of the suspension members may vary throughout the receiver coil assembly—for example, a greater number of elastic suspension members may be positioned at an intended top of the assembly than are positioned toward a bottom.
-
FIGS. 10B and 10C illustrates another possible suspension configuration of theinternal frame 12 forreceiver coil assembly 10. AlthoughFIGS. 10B and 10C illustrate theframe sections receiver coil 16A, theframe sections FIGS. 10B and 10C . The configuration shown inFIGS. 10B and 10C is similar to that described above in respect ofFIGS. 4 and 5 , with the addition of intermediaterigid rods 94 that run longitudinally incylindrical passage 22A between the opposite sides of theinternal frame member 12A and the wall defined by theouter frame member 14A. Theelastic suspension members 32 on one side of theinternal frame member 14A offrame section 20A are each attached at approximately amidpoint 98 to one of therigid rods 94, and theelastic suspension members 32 on the opposite side of theinternal frame section 20A are each attached at amidpoint 98 to the other of therigid rods 94. Therods 94 can tie the suspension members together to spread forces applied to any individual member among a number of suspension members. Additionally, therods 94 themselves can act as energy absorbing or reflecting structures in that the rods act as a mass or inertia that provides an additional stage of isolation for inner frame from the vibration of the outer frame. In one example embodiment,rods 94 extend substantially the length of therespective frame member 12A in which they are located. Therods 94 can in some embodiments be broken into smaller rod sections.FIG. 10C is further sectional representation, at right angles to the view ofFIG. 10B , illustrating the suspension system.Rods 94 can be formed for example from fiberglass or other composite materials or metal in some embodiments. -
FIG. 10D illustrates a further variation of the receiver coil suspension system ofFIGS. 10B and 10C . In the embodiment shown inFIG. 10D , theelastic suspension members 32 on each side of thereceiver coil 16A are formed from a single elastic member that extends in a zig-zag back and forth fashion between theinner frame member 12A and theouter frame member 14A.Rods 94 are secured at the midpoints of thesuspension members 32.FIG. 10 also illustratesrods 94 broken into smaller rod sections. - In addition to being used in a three receiver coil tow assembly as described above, the elastic receiver coil suspension systems shown in the figures and described above can also be applied to receiver coils used in other configurations, including for example single coil tow assemblies that include only a single receiver coil and double coil tow assemblies.
- Although octahedronal
receiver coil assembly 10 has been shown in the Figures, the skeletalreceiver coil assembly 10 could alternatively employ different shapes and arrangements of encased receiver coils. For example the skeletal frame could includes receiver coil frame sections that collectively define a skeletal spherical shape, or cubic, or pyramidal, for example. Additionally, more than three receiver coils could be used in some applications. For example,FIG. 15 shows an example of areceiver coil assembly 10′ that is substantially similar toassembly 10 except that the three orthogonal tubularsquare frame sections 8A′, 8B′ and 8C′ ofskeletal frame 8′ are connected to each other atmid-points 96 rather than at corners.FIG. 16 shows an example of anotherreceiver coil assembly 10″ that is substantially similar toassembly 10 except that theskeletal frame 8″ has a generally spherical profile and is formed from three intersecting tubularcircular frame sections 8A″, 8B″ and 8C″ that interconnect at points 96. - A description of example embodiments of a
receiver coil assembly 10 having been provided, some examples of how areceiver coil assembly 10 can be used will now be explained in greater detail. - In one example embodiment,
receiver coil assemblies 10 are used in the implementation of an airborne AFMAG geological survey system. Existing AFMAG based airborne geological survey systems typically operate based on the assumption that in the absence of local conductors the naturally occurring AFMAG magnetic field components measured above a surveyed terrain will have a random direction but lie in a horizontal plane, whereas the presence of a local conductor or a local magnetic body in the surveyed terrain will change the directions of the magnetic field components so that they are not horizontal. However, the assumption that natural magnetic fields are horizontal if there are no changes in subsurface conductance can introduce errors in some applications as the ionosphere conductors are not homogeneous and stable in time and can also cause changes of the audio-magnetic field vertical component. Error can also be caused by instability of the sensor coil assembly spatial attitude during a survey flight as any angular rotation of the sensor coils produces an equal error when measuring tilt angles. This error can be mitigated by using attitude sensors such as shown in U.S. Pat. No. 6,876,202 (Morrison et al.) but in some situations where base station measurements are used in combination with flight measurements the accuracy of error correction can be affected because of the unknown differences between the audio-magnetic field magnitudes in the base and flight points. Unlike typical AFMAG technologies, example embodiments are described herein in that do not rely on the relationship between vertical and horizontal components of the audio-magnetic field (or tilt angles) but rather on the relationship of the audio magnetic field 3D-vector magnitudes at two (or more) different points of the survey area at the same time. - Referring to
FIG. 11 , ageophysical prospecting system 110 according to one example embodiment of the invention includes at least two spatially separatedsensor systems sensor systems sensor system 112 includes a towed receiver coil assembly 10-1 suspended from atow cable 64 and towed by anaircraft 86 such as a helicopter or airplane or airship over a survey region, and theother sensor system 114 includes a ground based receiver coil assembly 10-2 that is stationary and located on theground 88. As will be explained in greater detail below, each of thesensor systems suspension coil assembly 10 described above in respect ofFIGS. 1-8 , or the doublesuspension coil assembly 10 described above in respect ofFIGS. 9 and 10A , or the suspension systems described above in respect ofFIGS. 10B-10D . - In at least some applications, the
geophysical prospecting system 110 is less sensitive to naturally occurring variations not caused by the presence of a conductor in audio-magnetic field tilt angles (for example variations caused by the ionosphere or changes in the surface terrain) and are not as dependant on the aircraft-towed sensor's spatial attitude as typical AFMAG systems. As known in the art, the Earth's natural electromagnetic field in the audio-frequency range can be observed and measured almost everywhere. The most stable frequency range that is least dependant on ionosphere conditions changes is typically 8-400 Hz, which is thus a convenient range for geophysical surveys. In some applications, the measurement range begins at approximately 25 Hz because motion-induced noise on an aircraft towed sensor assembly tends to be greater at lower frequencies, however in some configurations it may be possible to measure at frequencies lower than 25 Hz, such as 8 Hz, and higher than 400 Hz. When a region has an electrically homogeneous material beneath the earth's surface, the magnetic component of the alternating natural electromagnetic fields in the region will also be homogeneous. Thus, when a survey of the region is conducted using anairborne sensor system 112 and a stationary basestation sensor system 114, the 3D vector magnitude of the audio magnetic field measured at the stationary groundstation sensor system 112 and the aircraft towedsensor system 114 will typically be the same except for differences due primarily to the sensor system's 112, 114 spatial separation. In the case where underground conductors or other scattering bodies are present, the field intensity in a region is non-homogeneous, and varies throughout the region due to additional electromagnetic fields generated by the eddy currents induced through natural electromagnetic events in the underground conductors. - The differences in the 3D vector magnitude of the audio magnetic field measured at the stationary ground base
station sensor system 114 and the aircraft towedsensor system 112 will typically be greatest over an underground conductor, and this difference is used in thegeophysical prospecting system 110 to detect possible underground conductors. Thus, the peaks of the anomalies detected using thegeophysical prospecting system 110 are, in at least some applications, located over the top of underground electrical conductors, as compared to typical tilt-angle based AFMAG systems that rely on field tilt angles in which the peak anomalies occur at the sides of the conductors (e.g. at the transition between materials of differing conductivity). In at least some applications, thegeophysical prospecting system 110 described herein which detects anomalies over the tops of conductors may facilitate easier geological interpretation as the total field intensity anomalies may be stronger than the Z-component-only anomalies which are used in the known tilt-angle AFMAG systems, thereby providing a better signal-to-noise ratio. Furthermore, in at least some example embodiments the 3D vector field magnitude comparison system described herein does not require any attitude sensors which can simplify system instrumentation. - An overview having been provided, the airborne
geophysical prospecting system 110 will now be described in greater detail with reference toFIGS. 11 and 12 . In one example embodiment, the towed receiver coil assembly 10-1 of theairborne sensor system 112 includes threeelectromagnetic sensors axis coil 16A has a dipole axis that runs along a Z-axis, a second orX-axis coil 16B has a dipole axis oriented in an X-axis direction, and a third or Y-axis coil 16C has a dipole axis that is oriented along a Y-axis direction. The X, Y and Z-axes are fixed relative to receiver coil assembly 10-1 rather than any external reference and correspond to three lines that intersect at a common point substantially at the center of receiver coil assembly 10-1, with each axis being orthogonal to the other two axes. - In one non-limiting example embodiment, the orthogonal Z, X and Y sensor coils 16A, 16B and 16C are multi-turn air-core coils having a diameter of three meters and an effective area of 1000 square meters each; however other effective areas and other sensor shapes and configurations can be used. As will be appreciated from the above description of
FIGS. 1-10 , the receiver coil assembly 10-1 can in at least some applications maintain a substantially consistent coil area for each of thecoils - Analog signals that are representative of the natural magnetic field strength from the Z-
coil 16A, theX-coil 16B and the Y-coil 16C are provided through a connection box 108 (which may include a low noise amplifier) andcable 124 to an airbornedata collection computer 122 that will typically be located insideaircraft 86.Computer 122 includes an analog to digital converter device (ADC) 128 connected to receive the analog signals fromsensor coils ADC device 128 includes three 24-bit analog to digital converter channels (one for each measurement axis) for digitizing the analog signals from the Z-coil 16A, theX-coil 16B and the Y-coil 16C simultaneously. In one non-limiting example, the channels are each sampled at 2000 Hz, however other sampling rates can be used. The on-aircraft computer 122 is equipped with one or more storage elements that can include RAM, flash memory, a hard drive, or other types of electronic storage to store the digitized signals derived from the towed receiver coil assembly 10-1 and other input devices that are connected tocomputer 122.Computer 122 may be configured to perform data processing functions on the signals that it receives. - In example embodiments, the
airborne sensor system 112 or host aircraft includes a Global Positioning System (“GPS”)device 138 such that data obtained from the towed receiver coil assembly 13 can be correlated with a geographical position and a GPS time signal. In an example embodiment, the airborne sensor system also includes analtimeter system 136 connected to theairborne computer 122 in order to correlate the data obtained from the towed receiver coil assembly 10-1 with altitude measurements. In an example embodiment,altimeter system 136 includes an altimeter device that provides data about the relative altitude of the towed receiver coil assembly 10-1 above the actual survey terrain. It may also include a further altimeter device that provides an absolute altitude of the towed receiver coil assembly 10-1 above a fixed reference (for example sea level). - The
stationary sensor system 114 is similar to theairborne sensor system 112 but is configured to be placed on a stationary base point, and includes a ground based receiver coil assembly 10-2 that is substantially identical to the towed receiver coil assembly 10-1. In this regard, the ground based receiver coil assembly 10-2 also includes Z, X andY sensors - As with the
airborne sensor system 112, in thestationary sensor system 114, analog signals that are representative of the natural magnetic field strength from the Z-coil 16A, theX-coil 16B and the Y-coil 16C are provided through a connection box 108 (which may include a low noise amplifier) andcable 124 to adata collection computer 123 that will typically be located near the ground receiver coil assembly 10-2. The ground baseddata collection computer 123 includes an analog to digital converter device (ADC) 128 connected to receive the analog signals from the threesensor coils ADC device 128 includes three 24-bit analog to digital converter channels (one for each measurement axis) for digitizing the analog signals from the Z-coil 16A, theX-coil 16B and the Y-coil 16C simultaneously. In one non-limiting example, the channels are each sampled at a rate that is similar to the sampling rate used at the airbornesensor system computer 122. The ground baseddata collection computer 123 is equipped with one or more storage elements that can include RAM, flash memory, a hard drive, or other types of electronic storage to store the digitized signals derived from the ground receiver coil assembly 10-2 and other input devices that are connected tocomputer 123.Computer 123 may also be configured to perform further data processing functions on the signals that it receives. Thestationary sensor system 114 includes a Global Positioning System (“GPS”)device 138 such that data obtained from the ground based receiver coil assembly 10-2 can be correlated with a GPS time signal and in some cases, a geographical location. - In an example embodiment, the data collected by airborne
data collection computer 122 and the data collected by the ground baseddata collection computer 123 is ultimately transferred overrespective communication links 130, 132 (which may be wired or wireless links or may include physical transfer of a memory medium such as laser discs or flash memory cards) to a data processing system such as adata processing computer 126 at which the electromagnetic field data obtained from the receiver coil assemblies 10-1 and 10-2 ofsensor systems altimeter system 136 and the GPS data fromGPS sensors 138 associated with each of the air receiver coil assembly 10-1 and ground receiver coil assembly 10-24 can all be processed to determine if any anomalies exist that may be indicative of underground body of interest. In some example embodiments, some or all of the processing functions performed bydata processing computer 126 may be performed at one or both of the airborne or ground baseddata collection computers - In operation, the
airborne sensor system 112 can be flown at a substantially constant speed in a series of parallel lines over a survey area to make a series of measurements of the audio frequency range magnetic field in three orthogonal vectors. Simultaneously, thestationary sensor system 114 is located on the ground within the survey region to also make a series of measurements of the magnetic field in three orthogonal vectors. Thestationary sensor system 114 should, in at least some example uses, be placed a sufficient distance from any industrial electromagnetic field sources such as power lines so that natural audio-frequency magnetic fields dominate the signals received at the location of thestationary sensor system 114 and the residual industrial noise filtered in subsequent signal processing. For example, in one application the distance of thestationary sensor system 114 from a major power lines could be at least 3 Km. In example embodiments, there is no requirement for any special spatial orientation or attitude of the airborne or ground receiver coil assemblies 10-1, 10-2. For example, the X, Y and X axis of the airborne receiver coil assembly 10-1 do not need to be oriented in the same direction as the X, Y and Z axis of the ground based receiver coil assembly 10-2, and the orientation of the airborne assembly 10-1 can change during flight without adversely affecting the survey results. - Thus, as a survey of a region is conducted, the airborne
data collection computer 122 receives and stores a stream of digitized data that is representative of the naturally occurring audio frequency magnetic field Hz(air)(t) as measured by the airborne Z-axis sensor coil 16A, the naturally occurring audio frequency magnetic field Hx(air)(t) as measured by the airborneX-axis sensor coil 16B and naturally occurring audio frequency magnetic field Hy(air)(t) as measured by the airborne Y-axis sensor coil 16C. Each of the airborne magnetic field measurements is stamped with a GPS location and time information received from theGPS sensor 138, and with altitude information from thealtimeter system 138. At the same time, the ground baseddata collection computer 123 receives and stores a stream of digitized data that is representative of the naturally occurring audio frequency magnetic field Hz(ground)(t) as measured by the ground based Z-axis sensor coil 16A, the naturally occurring audio frequency magnetic field Hx(ground)(t) as measured by the ground basedX-axis sensor coil 16B and the naturally occurring audio frequency magnetic field Hy(ground)(t) as measured by the ground based Y-axis sensor coil 16C. Each of the ground based magnetic field measurements is stamped at least with time information received from theGPS sensor 138, and in some embodiments also with location information. Thus, each of the airborne andstationary sensor systems - At the
signal processing computer 126, the three channel data records from each of the airborne andstationary systems - In one example embodiment, frequency-domain processing is then performed on the data records either through applying narrow-band filters or applying Fast Fourier transforms on multiple consecutive time blocks (by way of non limiting example, time blocks could each be 0.5-2 seconds long), resulting in a time series of data that represents the magnetic field measured by each of the respective sensor coils at specific audio frequencies. This data includes a real and imaginary number representation of the magnetic field components for each of the X, Y and Z axes as measured in the air and on the ground. Using this information, the strength of the magnetic field at a particular frequency as measured at each of the airborne and ground sensors can be determined and compared for different locations in the survey region. Certain frequencies can be filtered out—for example 60 Hz noise is removed in some embodiments.
- By way of example, the strength of the magnetic field as measured at the airborne receiver coil assembly 10-1 at a particular frequency at a particular time can be represented as:
-
|H (air)(f)|=√(H z(air)(f)2 +H x(air)(f)2 +H y(air)(f)2) - and the strength of the magnetic field as measured at the ground based receiver coil assembly 10-2 at the same frequency and time can be represented as:
-
|H (ground)(f)|=√(H z(ground)(f)2 +H x(ground)(f)2 +H y(ground)(f)2) - The magnitudes of these vectors do not depend on the sensors' spatial attitudes, and by comparing the magnitudes of the vectors |H(air)(f)| and |H(ground)(f)| at different points of the survey flight, anomalies that are indicative of possible underground bodies of interest can be identified. By way of example, in the case of homogeneous terrain both the airborne and ground measured vector magnitudes will be substantially identical providing an airborne to ground ratio of equal or close to one. Deviation from this level can be treated as geophysical anomalies, with an airborne to ground ratio (|H(air)(f)|/|H(ground)(f)|) above one indicating a possible conductive underground (or scattering) body and below one indicating a less conductive body.
- Although the above example has focused on comparing the magnitude or strength of the magnetic field vector H(air)(f) measured by airborne sensor assembly 13 relative to the strength of the magnetic field vector H(ground)(f) that is measured at a spatially separated sensor assembly 15, in some example embodiments other features of the magnetic field vectors H(air)(f) and H(ground)(f) can be compared to determine if anomalies that are suggestive of a conductive or scattering body can be detected. For example, in addition to changes in relative magnitude of the vectors, changes in the relative phases or orientation of the magnetic field vectors H(air)(f) and H(ground)(f) can provide anomalies that are suggestive of a conductive or scattering body. Accordingly, in example embodiments one or more of the relative magnitude, phase and orientation of the magnetic field vectors H(air)(f) and H(ground)(f) can be compared to determine if an anomaly indicative of a conductive or scattering body has been recorded.
- In another example embodiment, the actual vector components are compared and the attitude-invariant properties of the relating tensors calculated. As soon as the electromagnetic wave is plane for any particular frequency, there is a coordinate system where the 3D magnetic complex vector {Mx, My, Mz} can be treated as a two 2D vectors, real and imaginary, for example Re(M)={Mu, Mv} having the third coordinate component equal to zero (except for the effects of noise). The new coordinate system can be found using known methods of vector-algebra by rotating the coordinate system and the resulting 2-D vectors then processed using adaptations of methods known for the processing of 2-D electric vectors. For example procedures for processing 2-D electric vectors are used in ground geophysics in magneto-telluric methods—sounding and profiling (see for example (1) Vozoff, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, 37,98-141. and (2) Anav, A., Cantarano, S., Cermli-Irelli, P., and Pallotino, G. V., 1976, A correlation method for measurement of variable magnetic fields: Inst. Elect. and Electron. Eng. Trans., Geosc. Elect., GE14, 106-1 14, which are hereby incorporated by reference). The procedures applied to electrical 2D vectors in these methods can be adapted and applied to the magnetic 2D vectors. One of the resulting parameters is a determinant of a matrix reflecting relations between the ground sensor system and in-flight sensor system derived magnetic vectors. This determinant is independent of the sensors' spatial attitude (invariant under rotation of the coordinate system) and it also reflects the relation between energies of total signals in the ground and airborne survey points. The determinant is also equal to one over homogeneous regions and deviates over any lateral conductivity changes.
- An alternative method for processing the signals received at the airborne and ground based sensors will now be described. According to another example embodiment, the airborne
geophysical prospecting system 110 measures the magnetic field in several frequency bands at the basestation sensor system 114 and at theairborne sensor system 112 and expresses the magnitude and phase of the magnetic fields as complex numbers. For example, these complex numbers could be the output of the frequency-domain processing referred to above. For each frequency band thesystem 110 estimates the 3×3 matrix which transforms the base station field into the airborne field. The estimates are calculated from the measured signals for each time interval of 0.4 to 1 s. -
A=TB -
- where A is a column 3-vector of complex numbers, the observed magnetic field at the
airborne system 112, in the airborne system frame of reference,- B is a column 3-vector of complex numbers, the observed magnetic field at the
base system 114, in a North-East-Down frame of reference, - T is the 3×3 transfer function matrix.
- B is a column 3-vector of complex numbers, the observed magnetic field at the
- where A is a column 3-vector of complex numbers, the observed magnetic field at the
- Since the
airborne system 112 attitude is arbitrary, the field observed there is rotated by an orthogonal matrix relative to the field that would be observed in a North-East-Down frame of reference. -
A=RT0B -
- where R is a 3×3 real orthogonal matrix which transforms a vector from the North-East-Down frame of reference to the airborne system frame of reference,
- T0=R−1T is the 3×3 transfer function matrix in North-East-Down coordinates.
- where R is a 3×3 real orthogonal matrix which transforms a vector from the North-East-Down frame of reference to the airborne system frame of reference,
- The objective is to define parameters of T0 that can be calculated from the known T, without knowing R. That is, the derived parameters must be invariant under rotation or changes in attitude of the
airborne sensor system 112. - To find conductive bodies, data is processed to identify locations where the source field (identified with B) is significantly amplified or attenuated. If there were just one component in B, the ratio of |A|/|B| would be a rotation invariant parameter of that kind. Since there are three components of B, a vector BM which gives the greatest amplification can be identified.
- The greatest possible amplification can be determined by singular value decomposition of T (See for example http://en.wikipedia.org/wiki/Singular_value_decomposition). A singular value decomposition is a set of matrices U, TS, V, which meet these criteria:
-
T=UTSV* -
- where U is a unitary matrix, i.e. U*U=UU*=I,
- TS is a diagonal matrix with real non-negative values,
- V is a unitary matrix, i.e. V*V=VV*=I,
- * denotes the conjugate transpose.
(In the case of a non-square matrix, “diagonal” means that the only non-zero elements are those for which the row number and the column number are equal, just as in the case of a square matrix.)
- where U is a unitary matrix, i.e. U*U=UU*=I,
- The non-zero diagonal elements of TS are singular values. If they are placed in decreasing order of magnitude, then TS is unique. (However, U and V are not unique.)
- Since T0=R−1T, it follows that
-
T0=U0TSV* - where U0=R−1U is also unitary matrix.
- So, the singular values of T and T0 are the same, hence they are invariant with respect to rotation of the
airborne system 112. (They are also invariant with respect to rotation of the basestation sensor system 114.) The singular value with largest magnitude is the largest amplification factor. The squares of the non-zero singular values of T are the eigenvalues of T*T. The singular value decomposition can be computed with standard publicly available software modules, e.g. function gsl_linalg_SV_decomp of the GNU Scientific Library, see http://www.gnu.org/software/gsl/. Since Bz is (in MT theory) dependent on Bx and By, the rank of T is at most 2 save for the effect of noise and errors, and so at least one of its diagonal elements should be negligibly different from zero. - Another option is to make T a 3×2 matrix, and compute the singular values accordingly. In the singular value decomposition, all the phase information is in U and V, which are not unique. The phase of the elements of U and V can in at least some situations be analyzed to get some meaningful information about the phase shift between the base station and the
airborne system 112. U and V can be separated into a unitary matrix which is in some sense “zero phase”, and a unitary matrix which is a diagonal matrix and contains the phase information. -
U=U0UP, V=V0VP -
- where U0, V0 are “zero phase” unitary matrices,
- UP, VP are diagonal unitary matrices.
- where U0, V0 are “zero phase” unitary matrices,
-
T=U0UPTSVP*V0*=U0TPV0* -
- where TP=UPTSVP* is a diagonal matrix which contains complex amplification factors, arranged in decreasing order of magnitude.
The phase of U0 can be minimized by this procedure:
- where TP=UPTSVP* is a diagonal matrix which contains complex amplification factors, arranged in decreasing order of magnitude.
-
- where Xij denotes the element of a matrix X at row i, column j.
- That is, U0 is obtained by multiplying each column of U by a unit magnitude phase factor so that the sum of each column of U0 is non-negative real.
- The identical procedure can be used to minimize the phase of V0. Other procedures are also possible. The procedure described yields three rotation invariant parameters, the diagonal elements of TP, which contain phase information and are related to the amplification of the primary field in the presence of subsuface geological structure.
- A more concise set of parameters is desirable in at least some applications. As previously noted, the rank of T (and therefore of TP) is at most 2 save for the effect of noise and errors. Since the elements of TP are in decreasing order of magnitude, the third diagonal element will be negligibly different from zero and can be discarded. To obtain a single rotation invariant parameter, the first and second parameters can be multiplied.
-
K=TP1TP2 - where TPi is a the i-th diagonal element of TP.
- In the absence of any subsurface features, the magnetic field is uniform everywhere. In that case, TP1=TP2=1, and therefore K=1. Over a conductor, it is expected that the amplification parameters will have magnitude>1, thus |K|>1. While the detailed behaviour of the parameter K will be complicated, in simplified terms the presence of a conductor will be indicated by a positive anomaly on a profile or map of the parameter K.
- A different (and perhaps simpler) algorithm can be used to obtain a related, though not identical, single rotation invariant parameter.
-
K=T 1 ×T 2. -
K′=K·Re(K)/|Re((K)| - where Ti is the i -th column vector of the 3×3 transfer function matrix T.
- The rotation invariance of this alternative parameter K′ is evident since the equality P=Q×R, the dot product Q·R, and the length (modulus) |Q| are preserved under any rotation of the basis of any vectors Q, R and their cross product P. The similarity to the parameter K is made evident by making the simplifying approximation that the unitary matrices U0, V0 are not “minimum phase” as defined above, but have no imaginary part and are therefore rotations or reflections. Since K′ is invariant under rotations (and may reverse sign under reflections), it follows that it can be computed from TP.
-
K=T P1 ×T P2=[0,0,T P1 T P2| -
K′=±T P1 T P2 =±K - where TPi is a the i-th diagonal element of TP.
- With the simplifying assumption that U0 and V0 have no imaginary part, parameter K′ is the product of the two non-zero amplification parameters, or its negative.
- The parameters K and K′ are example embodiments of the method of mapping subsurface structures, using natural sources and magnetic receivers, by calculating and displaying parameters that are independent of any rotation (including rotation about non-vertical axes) of a three axis
airborne sensor system 112 and/or a two or three axisbase sensor system 114. - It will thus be appreciated that in the low frequency magnetic field sensing embodiments discussed above, measurements can be made and used without regard to changes in attitude or rotation of the sensor systems. In this regard, the survey system is rotation invariant with respect to independent rotations of the
airborne system 112 and/or thebase station 114 about any axis, whether vertical or not. - In some applications of the
geophysical prospecting system 110, thecoils -
FIG. 13 illustrates another example embodiment of an airbornegeophysical prospecting system 200 that is similar in operation and configuration tosystem 110 except for differences that will be apparent from the Figures and the following description. In thesystem 200 ofFIG. 3 , thestationary sensor system 114 ofsystem 110 is replaced with a secondairborne sensor assembly 115 that is suspended from the same aircraft as the firstairborne sensor assembly 113 by arespective tow cable 202 that is longer than thetow cable 64. Although suspended from thesame aircraft 26 the first andsecond sensor assemblies second sensor assembly 115 being at a lower altitude. The functions of ground baseddata collection computer 123 and in at least some configurationsdata processing computer 126 are integrated intoairborne computer 122. In case of a homogeneous terrain, both3D sensor assemblies computer 122 and can be processed in the real time. - In some example embodiments, receiver coil assemblies 10-1 and 10-2 can alternatively be used in AFMAG-type geophysical prospecting systems that depend on tipper or tilt angle measurements as shown for example in above-mentioned U.S. Pat. No. 6,876,202. In such an application, attitude sensors can be located on the receiver coil assemblies 10-1 and 10-2 so that the orientation of such assemblies can be detected and the orientation information used in the calculation of tilt angle information that is derived from the signals collected from the receiver coil assemblies 10-1 and 10-2. For example, one or more accelerometers can be secured to the coil assemblies 10-1 and 10-2 to determine attitude information. Alternatively, three GPS receivers can be placed at spaced apart locations on the receiver coil assemblies 10-1 and 10-2 in order to track their respective attitudes.
- In some example embodiments, both the tipper measurement methods described for example in U.S. Pat. No. 6,876,202 and the 3-D vector processing methods described above can be incorporated into single
geophysical prospecting system 110, with the receiver coil assemblies 10-1 and 10-2 measuring the signals required for both types of calculations. - The
receiver coil assembly 10 could also be incorporated into active geophysical prospecting systems such as time domain electromagnetic (TDEM) geophysical survey systems or frequency domain electromagnetic systems (FDEM). By way of example thereceiver coil assembly 10 could be integrated into a TDEM system such as shown in U.S. Pat. No. 7,157,914.FIG. 14 shows a schematic view of an airborneTDEM survey system 200 that includes atransmitter coil 204 and a receiver coil assembly 10 (having 3-orthogonal coils TDEM survey system 200 can be carried by anaircraft 228 such as an airplane, helicopter, balloon or airship, for example. In at least some example embodiments, thetransmitter coil 204 andreceiver coil assembly 10 are part of atow assembly 212 that is towed by theaircraft 228. In the example embodiment shown inFIG. 14 , thetransmitter coil 204 and thereceiver coil assembly 10 are substantially concentric, with thetransmitter coil 204 being mounted to a frame that is suspended from multiple support cables orropes 216 that are each attached to a unique point on the circumference of the transmitter coil frame at one end and to acommon tow cable 215 at the other end. Thereceiver coil assembly 10 is centrally supported by a series of radially extending cables orropes 214 that extend to the transmitter coil frame. In one example embodiment, when in use thetransmitter coil 204 is horizontally positioned with a substantially vertical dipole axis, and thereceiver coil assembly 10 is located at a center of the transmitter coil 104, with the axis of thefirst receiver coil 16A being located in substantially vertical plane, the axis of thesecond receiver coil 16B being located in a substantially horizontal plane aligned in the direction of travel, and the axis of thesecond receiver coil 16B being located in a substantially horizontal plane aligned orthogonally to the direction of travel. - Measurements from the three receiver coils can be used to determine conductivity of bodies located in the survey region.
- It will be appreciated by those skilled in the art that other variations of the embodiments described herein may also be practiced without departing from the scope of the invention. Other modifications are therefore possible.
Claims (24)
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Also Published As
Publication number | Publication date |
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AU2009329786A8 (en) | 2014-07-24 |
AU2009329786B8 (en) | 2014-07-24 |
US8358135B2 (en) | 2013-01-22 |
RU2523106C2 (en) | 2014-07-20 |
RU2011126861A (en) | 2013-01-27 |
EP2380044A4 (en) | 2017-11-15 |
CN102265187B (en) | 2014-07-02 |
AU2009329786A1 (en) | 2011-07-28 |
CA2748278A1 (en) | 2010-07-01 |
BRPI0918195A2 (en) | 2015-12-01 |
CA2748278C (en) | 2015-09-22 |
WO2010071990A1 (en) | 2010-07-01 |
EP2380044A1 (en) | 2011-10-26 |
AU2009329786B2 (en) | 2014-06-26 |
CN102265187A (en) | 2011-11-30 |
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